The overcoat on a photoconductor can improve wear and erosion resistance and can mitigate crazing and lower the negative fatigue of the photoconductor drum. While numerous patents exist in the prior art (outlined below), no overcoat materials combine the advantages of wear resistance, fatigue improvements, and inhibition of crazing phenomenon.
In electrophotography, a dual layer photoconductor or photoreceptor is comprised of a charge generation layer (CGL) and charge transport layer (CTL) coated onto a suitable substrate, such as aluminized MYLAR polyester or an anodized aluminum drum. The CGL is designed for the photogeneration of charge carriers and is comprised of pigments or dyes, such as azo compounds, perylenes, phthalocyanines, squaraines, for example, with or without a polymer binder. The CTL layer, as its name implies, is designed to transport the generated charges. The CTL contains charge transport molecules, which are organic materials capable of accepting and transporting charge, such as hydrazones, tetraphenyl diamines, triaryl amines, for example.
Typically, the CTL also contains polymer binders, which are present to provide a wear resistant surface. Moreover, the polymer binders create adhesion between the layers and give a smooth surface, which can be easily cleaned.
As printers are made to perform at faster and faster print speeds, very short charge and discharge intervals are required. These faster speeds put increasingly greater demands on the PC drum and can shorten their effective useful life. In addition, the demand for smaller printer footprints puts additional constraints on the PC drum design. The PC drum may also be exposed to room light during servicing, which can cause fatigue in the PC drum.
Fatigue corresponds to the change in voltage over the life of the drum. In addition to fatigue from room light, fatigue can also result from drum cycling (repeated charge/discharge cycles) or from exposure to UV radiation, such as that emitted from a corona discharge lamp. Positive fatigue corresponds to photoconductor drums that discharge at lower voltages. For example, if a drum initially discharges to −100V, and on cycling or after exposure to room light discharges to −50V, the drum is exhibiting a positive fatigue of +50V. This positive fatigue would result in darker prints compared to the initial ones. Similarly, negative fatigue corresponds to a drum exhibiting a discharge voltage that is higher than the initial and would result in lighter prints.
Therefore, controlling the drum fatigue is important for the reproducibility of prints. The PC drum may also be more accessible to possible contamination from the environment or the user during routine maintenance. Furthermore, if smaller diameter drums are required because of space constraints, wear issues are magnified since more revolutions of the drum are required to print a page.
Silsesquioxanes have been incorporated into photoconductors as resin binders because of their abrasion resistant properties. Silsesquioxanes are compounds with the empirical chemical formula, RSiO1.5, and can be thought of as hybrid intermediate between silica (SiO2) and silicone (R2SiO). Sol-gel precursors are formed by the hydrolysis of trialkoxysilanes, which are cured to a mixed cage/network, or silsesquioxane structure.
When cured at higher temperatures, part of the cage structure is transformed into a more cross-linked network structure. Because of their cross-linked network structure, these materials are hard and have useful applications as abrasion resistant coatings, which include overcoats for organic photoconductor layers. Silsesquioxane layers are harder and less permeable to chemical contaminants than typical PC layers or binders such as polyesters or polycarbonates. Furthermore, these materials are known for low surface energy, which should make them good as release coatings to aid in toner transfer.
Silsesquioxane overcoats possess many other properties that are also advantageous for photoconductors. Because of their smooth surface, silsesquioxane overcoats are expected to increase the efficiency of particle transfer from the photoconductor surface, which is increasingly important as toner particle size decreases to meet the demands of higher image resolution. In addition to their smooth and hard features, these materials can also provide protection from physical, chemical, and radiation damage. For instance, the addition of acid scavengers to keep contaminants, such as acids, from reaching the photoreceptor surface. Likewise, dyes can be added to protect the photoreceptor from fatigue, especially from room light.
To address these issues to achieve a long life PC drum, a protective top layer can be coated onto the photoconductor drum. The protective overcoat can include additives that protect against damage from handling, exposure to UV light, and from the abrasion and erosion caused from the toner, cleaner blade, charge roll, for example.
While a robust overcoat can improve the life of the PC drum, a suitable overcoat is required that does not significantly alter the electrophotographic properties of the PC drum. If the layer is too electrically insulating, the photoconductor will not discharge and will result in a poor latent image. On the other hand, if the layer is too electrically conducting, then the electrostatic latent image will spread resulting in a blurred image. Thus, a protective layer that improves the life of the photoconductor must not negatively alter the electrophotographic properties of the PC drum.
The following references are illustrative of prior art employing silsesquioxane overcoats on photoconductors: U.S. Pat. Nos. 4,565,760 to Schank; U.S. Pat. No. 4,595,602 to Schank, particularly Example V, and U.S. Pat. No. 4,606,934 to Lee Et al, particularly Examples IX and XI.
U.S. Pat. No. 4,278,804 to Ashby et al discloses selesesquioxane combined with an ultraviolet light absorbing agent which is that employed in this invention, and U.S. Pat. No. 4,443,579 to Doin et al. discloses that agent chemically combined such that the material does not require a primer for overcoating. The material of this patent to Doin et al. is identical or substantially identical to the commercial material employed to practice this invention. Neither the Ashby et al. nor Doin et al. teaches overcoating a photoconductor. Silsesquioxane overcoats with UV absorbers have prevented the deterioration of polycarbonates from UV rays and are widely used in the automotive industry.